To date, various technologies for an electric power steering apparatus have been proposed. For example, Patent Document 1 discloses an integrated electric power steering apparatus in which a motor and a control apparatus that controls the motor are integrated with each other. Patent Document 2 discloses an electric power steering apparatus that continues driving of a switching device that controls a motor, for a time period from a time point when a control apparatus detects a suspicion of a short-to-ground failure to a time point when whether the suspicion of the short-to-ground failure is correct or incorrect is determined, and concurrently limits a voltage to be supplied from an electric power supply means to the switching device.
Patent Document 3 discloses an electric power steering apparatus in which while the electric power steering apparatus operates, the inherent resistance value of a motor is calculated so that when the rotation speed of the motor is the same as or lower than a predetermined value and the motor current is the same as or larger than a predetermined value, it is made possible to accurately perform decrease correction of a target current signal for driving the motor. Moreover, Patent Document 4 discloses a synchronous-motor control apparatus that includes an adaptive observer that calculates an angular frequency, an estimation current of a synchronous motor, an estimation rotor magnetic flux, and an estimation rotation speed, based on a current on a rotating 2-axis coordinates (d-q axes) and a voltage command on the rotating 2-axis coordinates (d-q axes) and an inverter that applies a voltage to the synchronous motor, based on the voltage command, and that configures the adaptive observer on the rotating 2-axis coordinates so that the synchronous motor can be controlled at a high rotation speed.
Patent Document 5 discloses an electric power steering apparatus that includes an electric-power supply unit that supplies to an AC motor high-frequency electric power on which high-frequency components corresponding to an inputted high-frequency signal are superimposed and a rotation position estimation unit that estimates a rotation position of the AC motor, and makes the rotation position estimation unit calculate the rotation position of the AC motor, based on the phase difference between an output-torque high-frequency wave included in the output torque of the AC motor and the high-frequency signal and hence accurately estimates the rotation position of the AC motor, without being restricted by the rotation speed, occurrence of magnetic-field magnetic saturation, or the like, so that the AC motor can accurately be controlled.
As described above, to date, technical development has been promoted from various points of view, and there have been proposed electric power steering apparatuses that are small-size, lightweight, and are capable of performing accurate control.